The temporal dynamics of leaf decomposition in a tropical stream were evaluated by measuring the input of leaves from riparian vegetation. The resulting mixture of leaves was processed by decomposer microorganisms and, in particular, by aquatic hyphomycetes, the most important microorganisms associated with leaf litter decomposition. Leaf decomposition and the characteristics and activity of the microbial community were investigated over the course of 1 year; these parameters depended on the composition and availability of leaves in the stream. We assessed the biomass, activity, and community structure of the fungi associated with the decomposing leaf mixtures monthly in a tropical stream with little seasonal variation in water characteristics. The leaf material included 27 riparian species; the amount of leaves of each contributing species varied from month to month, with higher overall values in the rainy season. Leaf decomposition was slow, and leaf mass loss did not occur monthly; however, leaf mass loss was significantly lower in May and September. The values of ATP, the ergosterol concentration, and the sporulation of aquatic hyphomycetes varied during the experiment, and only the ergosterol concentration (fungal biomass) was correlated with leaf mass loss. The sporulation rates of aquatic hyphomycetes peaked at the beginning of the rainy season (October), when only three hyphomycete species were present out of the total of seven that were found during the year. In this tropical stream, where the physical and chemical characteristics of the stream water varied little during the year, we hypothesize that changes related to leaf chemical composition could be the cause of changes in decomposition and the abundance and composition of the fungal assemblages. No significant differences were found for the parameters analyzed (lignin, cellulose, fiber, phosphorous, and polyphenols).